What is the tree-in-bud (TIB) characteristic of?

Tree-in-Bud Pattern: Characteristic Findings and Clinical Significance

The tree-in-bud (TIB) pattern on CT represents infectious bronchiolitis with mucoid impaction of small airways, most commonly caused by respiratory infections (72% of cases), particularly mycobacterial infections (39%), bacterial infections (27%), and aspiration (25%). 1, 2

Radiological Definition

The TIB pattern consists of centrilobular nodules (2-4mm) connected to branching linear structures that resemble a budding tree, representing dilated and inflamed bronchioles with mucoid impaction visible on high-resolution CT (HRCT). 1, 3 This pattern appears in peripheral lung regions and represents secondary manifestations of small airways disease. 1, 4

Primary Etiologies

Infectious Causes (Most Common)

Mycobacterial infections are the leading cause, accounting for 39% of cases with established diagnoses:

• Mycobacterium tuberculosis - the classic cause, particularly with endobronchial dissemination 4, 3
• Nontuberculous mycobacteria (NTM), especially Mycobacterium avium complex (MAC) 1, 2

Bacterial infections account for 27% of cases:

• Pseudomonas aeruginosa in bronchiectasis 1
• Acute bacterial bronchitis or pneumonia (present in 17.6% of these cases) 5

Other infectious agents include viral and fungal pathogens 3, 2

Non-Infectious Causes

Aspiration is the second most common cause overall, accounting for 25% of cases with established diagnoses. 2

Inflammatory disorders include:

• Diffuse panbronchiolitis 1, 4
• Inflammatory bowel disease-related bronchiolitis 6, 1
• Allergic bronchopulmonary aspergillosis (ABPA) - where centrilobular nodules with TIB appearance are common radiological findings that can occur in isolation or with bronchiectasis, mucus plugging, and high-attenuation mucus 6

Diagnostic Patterns and Associated Findings

Critical association: 96% of TIB cases (26 of 27) have associated bronchiectasis or proximal airway wall thickening. 5

Pattern Recognition for Specific Diagnoses

Random small airways pattern (alternating areas of normal lung with regions of TIB opacities and bronchiectasis):

• Highly specific (0.92) for Mycobacterium avium complex infection 2

Widespread bronchiectasis pattern (nearly uniform distribution):

• Highly specific (0.92) for diseases predisposing to airway infection: cystic fibrosis, primary ciliary dyskinesia, ABPA, immunodeficiency states 2

Bronchopneumonia pattern (consolidation with TIB opacities):

• Usually bacterial infection or aspiration 2

Aspiration-specific findings:

• Dependent distribution (specificity 0.79) 2
• Esophageal abnormality (specificity 0.86) 2

Temporal patterns:

• Chronicity strongly associated with mycobacterial infection (sensitivity 0.96, P<0.0001) 2
• Acute presentation associated with bacterial infection (specificity 0.87, P<0.001) 2

Tuberculosis-specific findings:

• Cavitations, especially in upper lobes or superior segments of lower lobes 4

Diagnostic Algorithm

Step 1: Imaging

• Obtain HRCT without IV contrast as the preferred initial modality 1, 7
• Look for mosaic attenuation on expiratory imaging (suggests air trapping) 1
• Identify cavitary lesions (suggest mycobacterial infection) 1

Step 2: Microbiological Evaluation

• Immediately collect sputum cultures for bacteria, mycobacteria, and fungi 1, 7
• If sputum studies are non-diagnostic, proceed to bronchoscopy with bronchoalveolar lavage (BAL) 1, 7
• In immunocompromised patients at risk for invasive aspergillosis, perform early BAL guided by CT findings 7
• For suspected invasive aspergillosis, obtain galactomannan from BAL fluid (cut-off ODI 0.5, sensitivity/specificity 88-90%/87-100%) 7

Management Principles

For NTM infection:

• Initiate macrolide-based multi-drug regimen for 12+ months once confirmed per ATS/IDSA criteria 1, 7
• Monitor with serial sputum cultures every 4-12 weeks during treatment 1, 7
• Obtain follow-up CT after completing treatment to document radiological response 1, 7

For bacterial infections:

• Administer prolonged antibiotic therapy targeted at the identified pathogen based on culture results 1, 7

For invasive aspergillosis:

• Voriconazole is first-line therapy (AII recommendation), with attention to drug-drug interactions and therapeutic drug monitoring 7
• Liposomal amphotericin B is an alternative option 7
• Consider combination therapy with voriconazole and caspofungin in high-risk patients 7

For non-infectious causes:

• Cessation of exposure, corticosteroids, and treatment of underlying conditions may be necessary 1

Critical Pitfalls

Do not delay bronchoscopy when sputum studies are negative or non-diagnostic, as this significantly impacts treatment decisions. 7

In immunocompromised patients, especially those with AIDS, tuberculosis may present atypically without classic patterns and may show only mediastinal lymphadenopathy or a deceptively normal chest radiograph. 4, 7

Do not overlook the 40% of cases where a definitive cause cannot be established despite thorough evaluation, requiring close clinical follow-up. 2